The present invention relates to an internal combustion engine intended for use with a fuel with a high vaporization heat either naturally (as alcohols, for example), or following hydration. It finds a particularly important application in the construction of engines, more especially industrial and agricultural engines, using fuels formed at least essentially by non petroleum products.
At the present time, practically all internal combustion engines use either the Beau de Rochas cycle with spark ignition or the self-ignition Diesel cycle (ignition by compression). None of these engines lends itself correctly to the use of the replacement fuels contemplated at the present time and, in the first place, alcohols. The use of alcohol as a fuel in an engine using the Beau de Rochas cycle results, even in the case of an engine with direct injection into the cylinders, in an insufficient efficiency and problematical operation particularly during cold start-up. The use of this same fuel in conventional Diesel engines, whose volumetric compression ratio varies particularly with the unit cubic capacity from 16 to 23 and in which the injection takes place at the end of compression, also leads to unsatisfactory results or to an impossibility because of the cetane deficiency.
FR Pat. No. 2 343 893 describes and claims an engine using a non petroleum fuel with fuel injection into the cylinder or each cylinder, having a volumetric compression ratio advantageously between 15 and 18, whose injection advance adjustment device is such that this injection advance is high, at least on startup. The use of a high advance, of at least 140.degree., possibly reaching a value of the order of 180.degree. during cold starts and a substantially higher value under normal operation, allows vaporization to take place during the compression phase because of heating by intimate mixture with the air admitted into the cylinder. It is indispensible to note in passing that the cycle thus obtained is totally different from that of the Diesel engine. There occurs in fact a veritable carburation effect during the compression phase, with intimate mixing and complete vaporization of the fuel in the pressurized air and at a high temperature, during the non adiabatic compression, before ignition. On the other hand, in the case of the Diesel engine, direct injection takes place with immediate combustion of the fuel sprayed into the air at high temperature, with a very small advance.
It is an object of the invention to provide an engine in which the combustion of high vaporization heat fuel takes place under satisfactory conditions and with a high overall thermal efficiency, cold starts remaining easy and the engine being able to have a general construction like that of a conventional Diesel engine.
To this end, there is provided an engine whose combustion chamber or each combustion chamber is defined by a piston driven with a reciprocating movement in a cylinder and a cylinder head closing the cylinder and supporting injection means. Means are provided for creating in the chamber a high turbulence of the intake air which results in the vaporization of the fuel during the compression phase, a stratification of the caburetted mixture, ready ignition and rapid combustion. These means comprise a recess having a rotational symmetry cup substantially of revolution formed in the dome of the piston disposed, with respect to the air intake means and, possibly, to the burnt gas exhaust means, so that the gases are set in rotation in the cup. The cylinder head carries in addition a sparking plug placed in the vicinity of the periphery of the cup, i.e. confronting a zone of the chamber where stratification creates a "rich" mixture. The injector supported by the cylinder head has advantageously several rows of holes, typically about ten holes, so that the fuel jet which it supplies is very well distributed. This arrangement may be compared to that of a Diesel engine, whose injector generally supplies a jet in the form of a sheet. The cup formed in the dome of the piston will generally have an approximately cylindrical shape off-centered with respect to the piston. If we designate by D.sub.0, D.sub.1 and H the diameter of the piston, that of the cup and the maximum depth of the cup, it will be generally advantageous to respect the following ranges: EQU 0.60.ltoreq.D.sub.1 /D.sub.0 .ltoreq.0.75 EQU 0.15.ltoreq.H/D.sub.0 .ltoreq.0.20
Thus, the swirling flow of the air intake causes a turbulence which is concentrated, when the piston is nearing TDC (top dead center), in the cup where a partial stratification occurs. With the spark plug placed close to the zone of maximum richness, there is rapid combustion in a small diameter chamber which allows, in turn, a small advance, resulting in a reduction of the thermal losses and a high expansion ratio. The cup is advantageously completed by a ramp whose angular development may for example be of about 180.degree., sloping in the flow direction of the gases in the cup, terminating in a steep side situated in the vicinity of the spark plug, which retains the last liquid particles. However, such a ramp is difficult to machine: a modification which is simpler to machine consists in using a stepped piston.
The invention proposes moreover simple means which may be used to avoid the dilution effect when the fuel is likely to have damaging effects on lubrication and/or pollute the crank-case through the piston rings.
The invention will be better understood from reading the following description of the elements concerned by the invention of an engine forming one particular embodiment thereof, given by way of example.